Kaolin is a naturally occurring, relatively fine, white clay which may be generally described as a hydrated aluminum silicate. Kaolin clay is widely used as a filler and pigment in various materials, such as rubber and resins, and in various coatings, such as paints and coatings for paper.
Slurries of kaolin clay are generally made by mixing a quantity of kaolin clay with water and other optional additives. However, chemicals added during beneficiation to achieve various desired results in these slurries, such as dispersants, colloidal thickeners, ammonia and leaching chemicals, also may provide usable energy sources for aerobic and anaerobic microbial growth. Because of these additives, most commercial kaolin slurries are susceptible to microbial spoilage and, therefore, require high levels of microbiocides to control aerobic and anaerobic microbial growth. Examples of such microorganisms include algae, bacteria, mold, spores, etc. The growth of microorganisms and their by-products tends to adversely affect the properties of a kaolin clay slurry by altering the color, odor and viscosity of the slurry. In many instances, the altered slurry may have little or no commercial value.
The problem of such microbial contamination in kaolin clay slurries is similar to problems in other industries, such as the pulp and paper industry or the petroleum industry, which have aqueous sources that encourage microbial growth. Microbial contamination can also significantly affect the efficiency of industrial structures or processes such as cooling towers or lubricating systems. Because of these problems, microbiocidal agents (i.e., microbiocides) are conventionally used, often at very high dosages, to eliminate or reduce the growth of microorganisms.
Microbiocides are well-known agents for the control of microbial growth in aqueous systems, such as kaolin clay slurries. However, the use of such agents raises environmental concerns which influence the choice of a microbiocide. Many of the early microbiocides were chlorinated or mercurial compounds which left harmful by-products in the environment. Some of the formerly used compounds, such as formaldehyde, are no longer environmentally acceptable because these compounds were found to be carcinogenic or teratogenic. These problems, therefore, limit the number of microbiocides available for use to those microbiocides which are less effective and more expensive.
There are a variety of problems associated with the use of microbiocides in the kaolin clay industry. These problems include the environmental impact associated with many microbiocides, the high cost of microbiocides and the large amounts of microbiocides that may be necessary to reach the desired microbial control level in a specific situation.
Microbiocides which are commonly used in kaolin clay slurries include 1,5-pentanediol; tetrahydro-3-5-dimethyl-2H-1,3,5-thiadiazine-2-thione; 1,2-benzisothiazolin-3-one; and 5-chloro-2-methyl- 4-isothiazolin-3-one/2-methyl-4-isothiazolin-3-one. The disadvantages to the use of these microbiocides include the high cost of their use at the necessary higher dosages, their ineffectiveness in certain slurries and their environmental impact.
As previously indicated, slurries of kaolin clay differ widely in their susceptibility to microbial contamination. Process additives are key factors but other factors, such as the degree of mechanical or chemical processing, variations in crude clay sources, differences in crude clay particle size distributions and crude clay impurities, may also create additional problems.
A method to solve some of the problems associated with microbial contamination is disclosed in Friedman et al. U.S. Pat. No. 4,975,109, which provides for using a combination of chemicals to act in a synergistic fashion to kill microorganisms. However, this patent requires the addition of an oxidizing agent, a microbiocide, a surfactant and an anti-corrosive material.
For various reasons, the prior art fails to provide an acceptable process that will control the level of microbial activity in a variety of kaolin clay slurries without the cost related to high dosages of the combinations of chemicals. Therefore, a need exists in the kaolin clay industry for an effective and broadly applicable process to reduce microbial growth and, therefore, provide commercially useful kaolin clay slurries having greater utility (i.e., a longer shelf life).